On fossil fuels such as petroleum and the like, there are a fear of future resource depletion and a problem of carbon dioxide emission causing a global warming phenomenon. In recent years, photo voltaic systems become widespread particularly due to growing environmental concerns and the cost-reduction of systems and are expected as alternative energy sources of fossil fuels such as petroleum and the like.
Common solar cells are classified into a bulk solar cell and a thin film solar cell. The bulk solar cell is made using a semiconductor of a bulk crystal such as monocrystalline silicon or polycrystalline silicon, or a gallium arsenide compound solar cell or the like, and in many of the solar cells, mass production technologies have been established. But, recently, there are problems that raw materials are lacking because of rapidly increasing production of the bulk solar cell and cost reduction is difficult. Contrary, since the thin film solar cell can reduce significantly the quantity of semiconductor to be used, it receives widespread attention as a next-generation solar cell which can resolve the shortages of the raw materials and has the potential for substantially reducing the cost. Specifically, while the above bulk solar cell has a thickness of several hundreds μm, the above thin film solar cell has a semiconductor layer of 10 μm to several μm or less. A structure of the thin film solar cell can be generally classified into the following two types; i.e., a superstrate type in which a transparent conductive layer, a photoelectric conversion layer and a backside electrode layer are stacked in this order on a transparent substrate and light enters a solar cell from a transparent substrate side, and a substrate type in which a backside electrode layer, a photoelectric conversion layer, a transparent conductive layer and a metal grid electrode are stacked in this order on a non-transparent substrate and light enters a solar cell from a metal grid electrode side.
In the thin film solar cell, since the quantity of a semiconductor used is small as described above, technology of making effective use of light entering the semiconductor layer is very important in order to attain high photoelectric conversion efficiency. One example of the techniques for this is an optical confinement technique. The optical confinement technique is a technique in which by forming a structure to scatter or refract light at the interface between the photoelectric conversion layer and a material having a refractive index which is different to that of the photoelectric conversion layer, a substantial optical path length in the photoelectric conversion layer is extended and thereby the quantity of light absorption is increased and the photoelectric conversion efficiency is enhanced.
Particularly in the thin film solar cell of the above-mentioned superstrate type, the above-mentioned transparent conductive layer is required to satisfy the following two effects in order to improve an optical confinement effect. First, light absorption in the above transparent conductive layer is low, that is, the transparent conductive layer has high transmittance. Thereby, the light entering the thin film solar cell from a transparent substrate side can be more sent into the photoelectric conversion layer. Secondly, the transparent conductive layer has a structure capable of scattering or refracting incident light effectively (an optical confinement structure). As this optical confinement structure, a surface texture structure of the above-mentioned transparent substrate, the above-mentioned transparent conductive layer or the like is often used. And, it is generally known that a haze index can be used as one characteristic for evaluating the above-mentioned optical confinement structure, and light scattered or reflected by this optical confinement structure increases as the haze index increases. Further, as the condition required of the above transparent conductive layer in addition to the optical confinement effect, it is also important that an electrical resistance (sheet resistance) is low. Since the above transparent conductive layer also serves as a collector electrode for collecting power generated in the photoelectric conversion layer and taking it out, when the sheet resistance becomes lower, a resistance loss is reduced and high photoelectric conversion efficiency can be attained.
Examples of conventional technologies using the above optical confinement effect include the following technologies. For example, in Japanese Unexamined Patent Publication No.2002-314109, a size or a density of a circular hole formed on the surface of the transparent conductive layer, and further a level difference between projections and depressions of a texture structure and a pitch of projections and depressions of a texture structure formed on the surface of the hole are specified. And, in Japanese Unexamined Patent Publication No.2002-141525, a root-mean-square of the level difference between projections and depressions of a texture structure on the surface of the transparent conductive layer, and an angle of tilt of projections and depressions of a texture structure are specified. However, when a substrate having a surface texture structure in which the level difference between projections and depressions of a texture structure is large and the pitch of projections and depressions of a texture structure is small is used, there are problems that mechanical or electrical defects resulting from the texture structure tends to occur and these defects causes the reduction in the open circuit voltage of the photoelectric conversion device or the reduction in the yield. Therefore, it is thought that this causes variations in the performance of the photoelectric conversion device to be larger. Then, Japanese Unexamined Patent Publication No.2000-252500 proposes to decrease defects of a photoelectric conversion unit layer and reduce variations in photoelectric conversion characteristics by specifying the level difference between projections and depressions of a texture structure of a first layer and that of a second layer, respectively, using a transparent conductive layer having a two layer structure
And, the use of a structure of a stacked photoelectric conversion device also constitutes a technology of making effective use of incident light. The structure of a stacked photoelectric conversion device is a structure for splitting an incident light spectrum and receiving the split light spectrum in a plurality of photoelectric conversion layers, and by stacking a plurality of photoelectric conversion layers which use a semiconductor material having a bandgap suitable for absorbing the respective wavelength bands in decreasing order of bandgap from light entrance side, it is possible to absorb the short-wavelength light in the photoelectric conversion layer having a large bandgap and the long-wavelength light in the photoelectric conversion layer having a small bandgap, respectively. Therefore, sunlight having a wider wavelength band can contribute to the photoelectric conversion compared with a device provided with one photoelectric conversion layer, and therefore it becomes possible to enhance the photoelectric conversion efficiency. Here, in the above-mentioned stacked photoelectric conversion device, since a plurality of photoelectric conversion layers are connected in series, an open circuit voltage becomes the sum of the voltages generated in the respective photoelectric conversion layers and makes effective use of these voltages, but the short circuit current density becomes limited to the minimum of photocurrents generated in the respective photoelectric conversion layers. Accordingly, equalization of the values of photocurrent generated in each photoelectric conversion layer is an important factor for making effective use of incident light energy. As a method of equalizing the values of photocurrent generated in each photoelectric conversion layer, a method of controlling a film thickness of each photoelectric conversion layer is common, but a method, in which the quantity of light entering each photoelectric conversion layer is controlled by providing an intermediate layer between adjacent two photoelectric conversion layers, is also known. When the above-mentioned intermediate layer is provided, part of light having reached the intermediate layer is reflected and the remainder of the light passes through the intermediate layer, and therefore the intermediate layer has an effect of controlling the quantity of light entering each photoelectric conversion layer, i.e., an effect of increasing the quantity of incident light entering a photoelectric conversion layer (top cell) on the entrance side of the intermediate layer and on the other hand decreasing the quantity of incident light entering a photoelectric conversion layer (bottom cell) on the opposite side of the intermediate layer. Characteristics of the intermediate layer to be desired are that an optical absorption coefficient is small at least in a wavelength region, light of which can be absorbed in the bottom cell and that the intermediate layer has an electrical conductivity of such a degree that a large series resistance is not produced, and materials satisfying these requirements are preferably used.
Further, as is disclosed in Japanese Unexamined Patent Publication No.2003-347572, when a surface configuration having an average pitch of projections of a texture structure within a range of 10 to 50 nm exists on the top face of the intermediate layer, there are not changes in effect of increasing the quantity of incident light entering a top cell and on the other hand decreasing the quantity of incident light entering a bottom cell, but the values of photocurrent generated in both the top cell and the bottom cell can be respectively enhanced by virtue of the optical confinement effect such as optical scattering, refraction or the like by the surface configuration at the top face of the intermediate layer. Thereby, the photoelectric conversion efficiency of a tandem thin film photoelectric conversion device is improved.